We are talking here GL350.
Yes, I know, I'm just saying that in the context of a discussion about DPF's, talking about heavy diesel approved oils designed for engines that also have DPF's isn't completely off the mark, it's at least somewhat germane to this discussion, particularly given that the OP is talking about running one of those lubes instead.
Mercedes wants approved oils and oils with that limit for a reason. We can talk forever about whether 0.8% or 1% or 1.15% matters, fact is the owner will get north of 150k DPF warning, exactly when depending on maintenance. I know BMW's with M57 engines running north of 200k easily on DPF, I racked up 485k km on original DPF. Yet, I know some who had to do cleaning around 150k miles.
Yes, Mercedes chose to base their approval on C3, which caps phosphorous at a lower level than CJ-4/CK-4. However, the OP is out of warranty and is clearly considering running an oil designed for a heavy diesel with DPF instead, that's the topic of the thread. So then the question becomes, how much impact will that really have on DPF longevity?
I own currently in a joint business with my brother 20+ commercial delivery vehicles, all small diesels in Europe, and only few were bought new. Some have DPF issues, some don't, some we maintain from the beginning. But whatever the issue is, we always use what the manufacturer wants us to use, which is mostly VW504.00/507.00. But eventually, DPF requires cleaning and we have business partner that does cleaning for us.
And that's fine. I'm running a Euro full-SAPS 0w-40 in my SRT, because I think it's a better oil, and has higher levels of AW additives than the "SN/GF-5" additive package used in the approved lubricant, and a Euro oil was originally the factory and service fill. Sometimes certifications and approvals change too, as you know, with BMW now back-spec'ing thinner oils for engines that originally called for an A3/B4 based lube. My M5 called for LL-01 back when M1 0w-40 and Castrol 0w-40 were approved, now neither of them are, would it be risky to continue using them, while the car is LONG out of warranty? Of course not.
And that brings us back to the EcoDiesel fiasco where they originally called for a Euro lube then switched to a CJ-4/CK-4. Clearly, they didn't think there was risk there. I ran Delvac 1 ESP 5w-40 in ours when we had it.
So yes, Mercedes calls for a specific lube here. The question is whether there is legitimate risk in him using an HDEO designed for larger diesels here instead, which he's already stated that he's doing. A good friend if mine did the same thing with his Jetta, running Delvac 1 in it instead of the 502 lube.